Technical Field
The present invention relates to a secondary cell state detector that detects a state of an electric cell such as a deterioration degree or an internal resistance of the electric cell.
Related Art
For example, a secondary cell such as a lithium-ion battery or a nickel-metal hydride battery is installed as a power source of an electric motor in various vehicles such as an electric vehicle (EV) driven using an electric motor, and a hybrid electric vehicle (HEV) driven using an engine together with an electric motor.
Such a secondary cell deteriorates through repeated charging and discharging, and a storage capacity (current capacity or power capacity) is known to gradually decrease. In addition, in the EV using the secondary cell, a storage capacity is obtained by detecting a deterioration degree of the secondary cell to calculate a distance at which the EV can be driven by the secondary cell, a life of the secondary cell and the like.
A state of health (SOH) corresponding to a ratio of a current storage capacity to an initial storage capacity is a factor that indicates the deterioration degree of the secondary cell. The SOH is known to have a correlation with the internal resistance of the secondary cell. If the internal resistance of the secondary cell is obtained, the SOH may be detected based on the internal resistance.
For example, a secondary cell state detector described in JP 2014-219311 A is proposed as a device that detects the internal resistance of the secondary cell. The secondary cell state detector of JP 2014-219311 A includes two condensers to hold respective cell voltages of two states of the secondary cell, for example, a discharge state and a discharge suspended state in the condensers, and amplifies a difference between the cell voltages held by the condenser using an amplifier circuit, thereby accurately obtaining an internal resistance or an SOH.
One plate of the two condensers of the above-described secondary cell state detector illustrated in JP 2014-219311 A is connected to ground, and a detection criterion is ground at all times. For this reason, when the secondary cell state detector illustrated in JP 2014-219311 A is applied to detection of states of secondary cells included in an assembled battery without change, a cell voltage of a connected secondary cell on a ground side is included in a measurement result except for a secondary cell closest to ground. Thus, there is a problem that accurate measurement cannot be performed.
Patent Literature 1: JP 2014-219311 A